1. Field of the Invention
The present invention is directed to the treatment of impure alunite in order to recover various chemical products therefrom, including pure alumina.
2. Description of the Prior Art
Much research has in the past been directed to the utilization of alumina-bearing minerals such as kaolinite, alunite, leucite, nephelite, muscovite, etc., since sufficient bauxite has not been available inthe industrialized countries for the manufacture of alumina. Of these alumina-bearing minerals, the clays exist in abundance in practically all countries, while alunite, which is represented by the formula Al.sub.2 (SO.sub.4).sub.3. K.sub.2 SO.sub.4. 4Al(OH).sub.3, is available in large quantities in the United States, Greece, and other countries. In addition, alunite contains other valuable components, e.g., potassium sulfate (K.sub.2 SO.sub.4) and sulfur trioxide (SO.sub.3).
With respect to alunite, the most serious impediment to the production of pure alumina therefrom has always been its high content of sulfur trioxide (SO.sub.3) which renders the application of alkaline methods of treatment (Bayer treatment, etc.) uneconomical. This is because the sulfur trioxide reacts with the alkali forming useless sodium sulfate (Na.sub.2 SO.sub.4); in other words, high losses of both sulfur trioxide and alkali are caused.
Efforts in the past have been made to remove SO.sub.3 from alunite prior to its treatment by alkaline methods. First of all, it was attempted to remove SO.sub.3 by the calcination of alunite at about 900.degree. C. by the following reaction (I): ##STR1##
Thus, alumina (Al.sub.2 O.sub.3) could be freed since the SO.sub.3 and H.sub.2 O formed sulfuric acid and the potassium sulfate (K.sub.2 SO.sub.4), being chemically free, could be dissolved and extracted in hot water. However, alunite is never found in a pure state; on the contrary, it is accompanied by substantial amounts of foreign matter such as SiO.sub.2, and these impurities remained in the washed residue of alumina after the removal of the filtrate containing the potassium sulfate. These silica and silica compound impurities rendered treatment and further purification of the produced alumina by alkaline methods uneconomical because the high temperature (900.degree. C.) applied during the calcination reaction reduced the solubility of the produced alumina in caustic solutions such as NaOH. This necessitated the use of stringent purification conditions, i.e., extended digestion in high concentrations of alkali under high temperatures and pressures, which then caused the co-dissolution of the accompanying free or combined SiO.sub.2.
Thus, it has been found that the contaminated alumina produced by the above reaction was of no higher value than white bauxite of high (and undesirable) SiO.sub.2 content.
Better results were obtained when alunite was thermally broken down at a lower temperature (500.degree.-590.degree. C.) and with the addition of potassium chloride (KCl) or sodium chloride (NaCl) or a mixture of both according to the reaction (II): ##STR2##
The alumina produced by this reaction (II) was more soluble than the alumina produced at 900.degree. C. (reaction I) and its dissolution in alkali could be effected under milder conditions, e.g., less concentrated solutions of NaOH, lower temperatures and use of atmospheric pressure. These conditions did not cause the undesirable dissolution of the free SiO.sub.2. However, in actual practice, since the SiO.sub.2 is rarely entirely free, but on the contrary a substantial portion of it is found combined with the alumina, consequently it would dissolve in the caustic solutions even under the milder conditions of concentration, temperature and pressure provided for by reaction (II).
Thus, this lower temperature method was successful only when the alunite was previously enriched to a purity of over 95% or when the accompanying gangue mineral consisted only of free, crystalline SiO.sub.2.
The present invention solves these prior art problems by applying a new technique in purifying the impure alumina produced in accordance with reaction (II) known to the prior art.